The majority of the world aluminium production takes place in electrolytic smelting cells employing the Hall-Heroult Process. In this process direct current is passed through a molten salt bath at a temperature of approximately 970° C. in which alumina is dissolved. The bath consists of a mixture of fluoride salts in which the main component is cryolite (Na3AlF6). As the electrolysis takes place in a molten salt at a high temperature in a corrosive environment, the service conditions for various electrical components of the electrolytic cell are arduous.
The molten bath in which the alumina is dissolved is contained within an electrolytic cell. The typical electrolytic cell comprises a rectangular steel shell lined with refractory materials as insulation, and carbon on the hot face. The carbon blocks on the bottom of the cell contain embedded conductors for the collection of current and act as the cathode. Carbon anodes are suspended from above the cell and dip into the bath. Metallic conductors known as anode assemblies or anode rods provide the mechanical support and carry the current to the anodes. The current design of the anode assembly is based on a steel structure attached to a carbon anode block. The steel structure is connected to the electrical bus bar via a copper or aluminium bar. The overall electrical resistance of a conventional anode assembly comprises the anode bar ohmic resistance, the steel structure (yoke) ohmic resistance and transition resistance between the steel structure and the anode bar.
During cell operation the bath is kept molten by the heat generated by the passage of electric current. The anodes are covered by a mixture of alumina and crushed bath to protect the anodes, particularly the connection points between the assemblies and carbon from airburn. During the process oxygen is released at the anodes where it reacts with carbon to produce mainly CO2 gas and release small amounts of CO and SO2.
In order to add alumina to the cell, the protective crust is first broken and the alumina added through the hole in the crust. As the fresh alumina contains a small amount of moisture in the alumina, fluorides and chlorides in the molten bath releasing a cocktail of gases (SO2, CHI an HF) which at elevated temperatures can be highly corrosive with respect to anode assemblies. The CO2 gas released at the electrolytic face is highly oxidising with respect to the consumption of carbon on the sides of the anode and on the hot faces of the anode which are exposed to the pot atmosphere below the crust and ore cover. This consumption of carbon from the anode sides reduces the life of the cell and represents an additional cost to the process.
Conventional aluminium reduction plants require a large infrastructure, typically costing above US$4000 per tonne of installed capacity and a large amount of electrical energy and carbon. The arduous service conditions within the cell impose expensive maintenance requirements on pots and anode assemblies. By increasing the production capacity of the existing plants and reducing the consumption of electrical energy and carbon and by reducing the need for anode assembly maintenance, a reduction of cost of aluminium production can be achieved.
An improvement on the conventional anode assembly is shown in U.S. Pat. No. 5,538,607. U.S. Pat. No. 5,538,607 discloses an anode assembly comprising an anode bar of high electrically conductive material. The end of a leg of the anode bar is received within a steel sleeve or stub and the stub inserted into a carbon anode block. While the anode assembly of U.S. Pat. No. 5,538,607, in theory is able to provide a high electrically conductive anode assembly which is easily maintained, the design does not address the practical problems faced in the application of an electrolytic cell such as oxygen burn out or anode block submersion.